The present invention relates to a method for making a radiation curable silicon containing polyacrylate hardcoat composition. Vacuum devolatilizing under anaerobic conditions is provided of an organic solvent mixture of aqueous colloidal silica modified with an alkoxysilylacrylate and polyfunctional reactive acrylate in the presence of an anaerobic gelation inhibitor. A substantially volatile free radiation curable silicon containing polyacrylate hardcoat composition also is provided.
As shown by Chung, U.S. Pat. No. 4,478,876, a process is provided for applying a UV curable silicon containing polyacrylate hardcoat composition onto a substrate, such as a thermoplastic substrate. The UV cure of the Chung silicon hardcoat composition is effected in a non-inert atmosphere, such as air.
In the preparation of a UV curable silicon containing polyacrylate hardcoat composition, aqueous colloidal silica can be initially treated with an alkoxysilylacrylate such as methylacryloxypropyltrimethoxysilane. Water is then removed from the mixture in the form of an aqueous/organic solvent azeotrope. Prior to devolatilizing, a polyfunctional reactive acrylic monomer, such as hexanedioldiacrylate, can be added to the mixture to provide an acrylic matrix for the silylated colloidal silica. Experience has shown that gelation of the silicon containing polyacrylate hardcoat mixture can occur readily during the distillation step. An inhibitor, such as methylhydroquinone (MEHQ) has been used with some degree of success. However, as taught by Kurland, J.Poly. Sci.,Poly.Chem. Ed., 18 (1980) 1139, inhibitors such as MEHQ require oxygen to be effective. As a result, safety concerns have arisen concerning the distillation of large volumes of organic solvent at elevated temperatures in the presence of air or oxygen.
It would be desirable therefore to be able to produce radiation curable silicon containing polyacrylate hardcoat compositions in an economic and environmentally safe manner. As used hereinafter, the expression"radiation curable" means cure resulting from actinic radiation such as UV light, or particle radiation such as electron beam.